This invention relates to an injection molding machine which has an injection molding unit displaceable on horizontal guides by a hydraulic drive assembly. The injection molding unit includes a plasticizing unit which has a plasticizing cylinder accommodating a feed screw which is rotatable and axially shiftable therein. The plasticizing cylinder is provided at its frontal end, that is, at its discharge terminus, with either a continuously open nozzle or a shutoff nozzle which can be opened or closed by a control device. At the opposite, rear terminal portion the plasticizing cylinder is releasably locked into a central opening of a carrier block of the injection molding unit. For purposes of replacement the plasticizing cylinder may be removed from the central bore in an axial direction relative to the carrier block by a hydraulic drive.
Injection molding units of the above-outlined type are known and are disclosed, for example, in U.S. Pat. No. 4,708,622, issued Nov. 24, 1987. Such injection molding units may operate with a plasticizing cylinder which may have either an open nozzle or a shutoff nozzle because a nozzle exchange may be carried out without appreciable loss of time within the framework of an automatic replacement of the plasticizing cylinder.
In many cases open nozzles are preferred because of their substantial versatility in connection with the use of various synthetic materials and because of their disturbance-free operation. They find application in connection with decompression devices which, by virtue of an axial reverse stroke of the feed screw, relieve the plastic material in the plasticizing cylinder from pressure prior to removing the plasticizing cylinder from the remaining components of the injection molding unit. This prevents an undesired escape of the plastic material from the nozzle.
Shutoff nozzles are required for special tasks in injection molding technology, for example, for the processing of polyamides. Such nozzles make possible, for example, a high injection velocity in the initial phase of the injection process when the plastic material in the plasticizing cylinder is accordingly pressurized prior to opening the nozzle.
Furthermore, it is known, for example, from French Pat. No. 1,439,001 or German Utility Model Pat. No. 7,020,969 to control the shutoff needle of the shutoff nozzle by a two-armed lever with the aid of a hydraulic power cylinder which is secured directly to the plasticizing cylinder in the zone of the shutoff nozzle.